1. Field of the Invention
The present invention relates to an automobile fuel tank which has good impact resistance as well as good impermeability to automobile fuel, especially gasoline blended with methanol or methyl t-butyl ether (MTBE).
2. Description of the Prior Art
Plastics fuel tanks are coming into general use. Most popular among them are those of single-layer type made of polyethylene. Their disadvantage is a comparatively high permeability to gasoline. Conventional measures against this were sulfonation of the polyethylene tank (Japanese Patent Publication No. 23914/1971), fluorination of the polyethylene tank (Japanese Patent Publication Nos. 21877/1972 and 15862/1978), and blending of polyethylene with nylon as a barrier material (Japanese Patent Laid-open No. 296331/1992). There has also been proposed a plastics fuel tank of five-layer construction in which the inner and outer layers are made of high-density polyethylene and the middle layer is made of nylon and they are bonded together with adhesive resin layers.
It has been found that these prior art technologies do not cope with the recent situation arising from the use of the gasoline which contains oxygenated compounds such as methanol, ethanol or MTBE (referred to as oxygen-containing gasoline hereinafter) which started in the U.S. to prevent air pollution, to reduce gasoline consumption, to improve the octane number of gasoline, and to lower the amount of unburnt hydrocarbon in exhaust gas. Unfortunately, the above-mentioned sulfonation or fluorination of the single-layered polyethylene fuel tank does not provide sufficient barrier properties for oxygen-containing gasoline. Likewise, the combination of nylon with polyethylene (in the form of multi-layer or dispersion) does not completely prevent permeation of oxygen-containing gasoline through the fuel tank.
To address this problem, there has been proposed a multi-layer fuel tank composed of polyethylene and ethylene-vinyl alcohol copolymer (EVOH for short hereinafter) which exhibits good gasoline barrier properties. Although it is superior in barrier properties to the conventional fuel tanks, there still is room for improvement to meet the future environmental regulations. Improvement in impact resistance is another subject to be considered.
One way to approach problems involved in the fuel tank having an intermediate layer of EVOH is by blending EVOH with polyolefin or polyamide (as proposed in Japanese Patent Laid-open Nos. 218891/1994 and 52333/1995). However, the problems still remain unsolved because such blending appreciably impairs the gasoline barrier properties and adversely affects the melt stability of the raw material.